Rates problem

GRE Quant Problem Solving
1

Not really understanding how to solve the following problem:
There are 4 card-processing machines in an office. The fastest of these machines processes x cards in 7 hours and the slowest processes x cards in 8 hours. which of the following (7. 2, 7.3, 7.5, 7.6, 7.7) could NOT be the average time per machine for each of the 4 machines to process x cards.

I tried doing the min and max as follows:
min: (3(x/8)+x/7)/4=(29x/56)/4
max: (3(x/7)+x/8)/4=(31x/56)/4

Stuck here.

2

Think about the extremes; don’t try to use algebra.

For example, show that the smallest possible average happens when the machine processes 7,7,7,7 and 8 cards.